1. Field of the Invention
The present invention relates to satellite acquisition, and more particularly to a system and technique for rapidly and accurately orienting an antenna to acquire optimum strength signals transmitted from a satellite selected from among a number of satellites available.
2. Problem to be Solved
In order to achieve maximum reception by an antenna of transmissions from an orbiting Earth satellite, it is important that the antenna be aimed directly at the satellite. This process is known in the art as "acquiring the satellite". The specific aiming requirements for different satellites vary, but if the direction in which the antenna is oriented differs from the optimum orientation for acquiring the satellite then suitable reception of the satellite signal by the antenna is not achieved. The acceptable deviation may be no more than a fractional degree in some military satellites and between one and a half and two degrees in some commercial satellites. Even when using antennas designed for satellite television reception, which do not have very demanding accuracy requirements, the antenna must be aimed within a few degrees of the desired satellite in order to achieve adequate reception.
The process of acquiring orbiting satellites is typically slow and tedious, even though there are satellite log books which provide the exact position of these satellites in terms of azimuth and altitude, or alternatively in latitude, longitude and altitude, relative to certain locations on Earth. Since most antennas are not located precisely at these Earth locations, when using such log book information, the person or device that is acquiring a satellite usually has to determine the satellite's position with respect to some other or new location remote from the location selected in the log books, and then align the antenna with the position of the satellite within the specified accuracy. Precise computation of where the antenna is directed relative to the satellite is difficult to perform. It can take a person trained in satellite acquiring a significant portion of an hour to acquire a single satellite when seeking satellites which must be acquired by an antenna within a more limited angular range. For the untrained acquirer, the process is likely to be an exercise in futility. As the process proceeds, the actions of the person attempting to acquire a satellite often become more disjointed, which reduces the probability of success occurring within a reasonable time. Satellite acquisition is one of the primary difficulties associated with satellite antenna usage.
There are many prior art acquisition processes. One of these is referred to as the "step track" acquisition process, in which the user initially coarsely acquires the satellite by orienting the acquiring device in the general direction of an omni-directional beacon signal (ADF) which emanates from the satellite. As soon as the acquirer roughly locates the satellite beacon signal, then some scan technique is used to detect the direction within the coarse acquiring region that the transmission signal from the satellite is most strongly received. Most known step track acquisition processes take a relatively long time to acquire a satellite.
Alternatively, in certain prior techniques, it is known to define a relatively large two dimensional angular range within which the satellite is located. As soon as the outside constraints of the angular range are determined, again some scanning pattern can be applied to determine the region where the satellite signal is received most strongly. Many scanning methods can be used to finely acquire a satellite, such as the stepping, raster-scan, conical-scan, or box-scan techniques.
In these prior art techniques, the original constraints used in coarsely acquiring the satellite are usually so large that a relatively long time is required for the ultimate scan to achieve fine acquisition. It is desirable therefore to more precisely define these constraints so that less time is required for the scan, and/or the scan can be concentrated in a smaller area to yield a more precise satellite signal acquisition.
In another satellite acquiring technique, multiple Global Positioning System (GPS) antennas, with each antenna attached to a distinct GPS sensor, are arranged about the periphery of a platform, such as a table. Each GPS antenna-sensor combination can precisely measure the distance to the satellite being acquired. A computer, with a distance-measuring algorithm, can then use these distances to precisely measure the relative position of the satellite with respect to the platform. With this approach the computer must utilize a relatively complex algorithm to acquire the satellites, and it is also necessary to use a plurality of GPS antennas and sensors.
While the foregoing acquisition processes are especially applicable to geo-stationary satellites, i.e., those with orbits that maintain them above a particuler location on the Earth, it is also possible to use such systems in conjunction with what are called tracking satellites, such as low earth orbit satellites (LEOS), the orbits of which vary their positions relative to the Earth. It is only important that an acquiring system be able to acquire such a satellite at a given time and place. After a tracking satellite is acquired, a tracking system in the acquiring antenna can be used to maintain contact with the satellite. The time constraints presented by tracking satellites, which are only going to be in a certain region of the sky for a relatively short period, makes it even more desirable to be able to quickly acquire these satellites. Similarly, in many other applications, especially many critical military and commercial ones, the acquisition must be achieved within a reasonable period. However, rapid acquisition is unlikely to be reliably achieved using prior art techniques. Therefore, in some applications where satellite communications would be superior to what is presently being used, they are not applied since the acquisition process is uncertain and slow.
From the foregoing considerations, it is apparent that a technique which would achieve rapid and reliable acquisition of satellite transmissions by antennas would be very useful and desirable in many commercial and military satellite applications. Also, an acquisition system would be desirable that is comparatively uncomplicated to operate and readily portable offering versatility of use.
Objects:
It is accordingly an object of the present invention to provide a satellite acquisition system and technique that will rapidly and accurately pick up satellite transmissions with maximum signal strength.
It is a further object of the present invention to provide such a system that is self-contained, without the need for multiple antennas, and capable of being hand-held and of being utilized with any satellite.